1. Field of the invention
This invention relates generally to hydraulic fluids and, more particularly, to hydraulic oils for hydrostatic power transmission.
Hitherto, most hydraulic oils have been made from mineral oil. In applications where the escape of oil into the environment is a possibility, for example through unavoidable leakages, there is an increasing demand for hydraulic oils which contain environment-friendly ester oils, particularly those based on rapeseed oil and/or soybean oil, as the oil base. Typical applications of the type in question are forestry and agricultural machinery and equipment, dredgers and the like. Applications such as these now require the use of hydraulic oils of water hazard class O. Ester-based hydraulic oils are capable of satisfying these requirements.
2. Discussion of Related Art
However, the above-mentioned ester oils essential for practical application, i.e. rapeseed oils and/or soybean oils which have been purified and, in particular, freed from amylopectins and other mucilaginous substances, have two crucial weaknesses in practical application, namely:
Ester oils based on unsaturated fatty acid systems tend to thicken rapidly, even at only moderately elevated operating temperatures of, for example, 50.degree. to 80.degree. C. This is attributable to the readiness of the olefinic double bonds of the ester-forming acids of the type of oil in question to enter under the effect of atmospheric oxygen into reactions which ultimately lead to an increase in viscosity. Although it is basically known that unwanted increases in viscosity can be counteracted by addition of antioxidants, it has been found that the antioxidants hitherto preferably used in hydraulic oils based on mineral oils do not perform satisfactorily in ester oils of the type in question.
Another important limitation for hydraulic oils based on the environment-friendly ester oils mentioned above is their inadequate stability at low temperatures. Purified rapeseed oil, for example, has a solidification point or pour point of -16.degree. C. Even before the solidification point is reached, the oil undergoes a significant increase in viscosity with falling temperatures. The comparatively high solidification point of rapeseed oil, for example in winter conditions, presents considerable problems for the practical application of the hydraulic oils at low ambient temperatures. In practical application, these problems can of course be dramatically aggravated by the fact that oxidative thickening of an ester oil of the above-mentioned type is accompanied by a considerable reduction in the solidification point of the hydraulic oil. The addition of pour point depressants does not solve the technical problem involved. It is known that the effect of pour point depressants disappears in the event of prolonged residence in the oil to be treated.